Thermoplastic materials, such as PET (polyethylene terephthalate), have been used for some time now to make containers. This is particularly true for food-containing containers, especially drinks. Although said containers can be of different types, they shall be referred to hereinafter generically as bottles, which happen to be the most spread.
Bottles made of thermoplastic materials are definitely convenient in terms of weight, resistance to impact, cost, and similar but also have some drawbacks. For example, said materials are somewhat microporous making, along with the limited thickness of the wall, the bottle permeable to gas. For example, oxygen may penetrate into the bottle modifying the contents through oxidation, and/or carbon dioxide found in many carbonated drinks may escape making the drink less fizzy and attractive.
Many solutions have been put forward to resolve these problems. Firstly, one solution involves increasing the thickness of the bottle wall; unfortunately, this increases production costs and can lead to problems during manufacturing. Secondly, one solution involves using multilayer bottles; however, this increases production costs and complexity. Thirdly, another solution entails depositing a thin layer to act as barrier on the internal wall of the bottles; unfortunately, this also increases costs and complexity.
An apparently simple and effective solution exists to the problem of creating a protective coating capable of acting as barrier to gas exchanges: painting, especially through dipping, the external surface of the bottles.
For example, patent U.S. Pat. No. 5,658,619 describes a process for coating bottles. This process involves sending bottles to a coating segment where the bottles are gripped and dipped one at a time in one of many containers filled with a coating solution consisting of a resin dispersed in a solvent. Then, after removing the bottles from the coating solution, the bottles are released and sent to a flash-off segment where the solvent of the coating solution is removed from the coating applied to the outer surface of the bottle. After the flash-off process, the bottles are sent to a reticulation station where the resin of the coating is reticulated.
Such a plant is complex and has a few critical points, especially regarding the formation of paint sagging in the detearing phase, during the transfer from the painting station to the flash-off station. Furthermore, eliminating the solvent found in the paint through simple flash-off is a long process that is not well controlled.
Sometimes paints with a water-based solvent are used to reduce costs and limit environmental pollution. Unfortunately, this option hampers drying, requiring long drying times or heating of the bottles at a temperature apt to promote the quick elimination of the solvent. If high outputs are required, said temperature is very near to, if not greater than, the temperature for softening the thermoplastic material of the bottles.
Hence, it is very important to provide a paint-drying system that limits any damage to the bottles and, at the same time, assures treatment methods and limited drying times, after painting, that prevent irregularities in the thickness of the coating.
A simple way of drying said water-based paints is to heat them; for example, by exposing them to infrared radiation (IR).
An infrared-heating plant is described, for example, in patent application PCT/EP00/10540, of this Applicant, although it refers to a plant for conditioning pre-moulded workpieces to be sent to final moulding, meaning that it is used to raise the temperature of said workpieces to one suitable for final moulding. In the aforementioned document, the pre-moulds are conveyed past a series of IR lamps; at the same time, an adjustable airflow at ambient temperature flows, first, around the pre-moulds and, then, around the IR lamps to cool them.
Although this solution is appealing, it concerns non-painted pre-moulds, which must simply be heated at an established temperature using different handling methods and short heating times.